US Navy
The Navy is still building and fielding its advanced Virginia-class attack submarines.
But the Navy is already planning for their eventual replacement by the SSN(X), or the Improved Virginia-class.
The United States Navy's Virginia-class fast-attack submarines are among the youngest in the Navy.
They're nuclear powered and armed to the teeth with four torpedo tubes as well as Tomahawk cruise missiles and Harpoon anti-ship missiles. The Virginias incorporate a number of new technologies and are among the stealthiest submarines in the world.
The Navy designed the Virginias from the outset with a number of features not previously seen in other submarines, such as a fiber optic fly-by-wire control system which replaced the mechanical control system seen in previous submarines, as well as a secretive electromagnetic signature reduction system that helps keep the submarine undetected.